Molecular biology
Like structural homologies, similarities between biological molecules can reflect
shared evolutionary ancestry. At the most basic level, all living organisms share:
• The same genetic material (DNA)
•
The same, or highly similar, genetic codes
• The same basic process of gene expression
(transcription and translation)
•
The same molecular building blocks, such as
amino acids
These shared features suggest that all living things are descended from a
common ancestor, and that this ancestor had DNA as its genetic material, used
the genetic code, and expressed its genes by transcription and translation.
Present-day
organisms
all
share these features
because
they
were "inherited" from
the ancestor (and because any big changes in this basic machinery would have
broken the basic functionality of cells).
Although they're great for establishing the common
origins of life, features like
having DNA or carrying
out transcription and translation are not so useful for
figuring out
how
related particular organisms
are. If we want to determine which
organisms in a group are most closely related, we need to use
different types of
molecular features, such as the nucleotide sequences of genes.
Homologous genes
Biologists often compare the sequences of related genes found in different
species (often called
homologous or orthologous genes) to figure out how those
species are evolutionarily related to one another.
The basic idea behind this approach is that two species have the "same" gene
because they inherited it from a common ancestor. For instance, humans, cows,
chickens, and chimpanzees all have a gene that encodes the hormone insulin,
because this gene was already present in their last common ancestor.